In a typical commercial nuclear power plant such as a pressurized water reactor (“PWR”), a number of components and systems are installed on or directly over the reactor vessel closure head. These components and systems may include one or more of the following: a control element drive mechanism (“CEDM,” also referred to as a control rod drive mechanism); a cooling system; a lift rig for the reactor vessel closure head; CEDM seismic restraints; and a CEDM missile shield. The components and systems are typically designed and installed individually to perform designated functions during plant operation.
It is well known that removal and subsequent re-installation of the reactor vessel closure head, including the requisite removal of various components disposed about the reactor vessel, is an expensive and time-consuming process. During refueling of the reactor, the installed components are generally disassembled from the reactor, removed and stored, to provide access to the reactor vessel closure head, so that the vessel head may be removed from the reactor vessel. The disassembled components are placed in designated storage areas, generally inside the reactor containment. Typically, in a commercial nuclear power plant, a lengthy series of steps or detailed procedures must be followed to safely remove external such equipment before the reactor vessel closure head is removed from the reactor vessel. The procedures that are performed prior to detensioning the reactor vessel closure head studs will generally include some or all of the following:                Removal and storage of heavy concrete missile shields;        Removal and storage of the CEDM cooling ducts;        Removal of the seismic restraints;        Disconnecting and storage of the CEDM power and rod position indicator cables;        Installation of the reactor head lifting rig tripod;        Removal of the cable trays and cables that extend from the reactor vessel closure head to the operating deck or walls;        Disconnecting the heated junction thermocouples, nuclear steam supply system instrumentation, monitoring system cables, and reactor head vent lines; and        Installation of temporary radiation shield blankets around the vessel closure head area.        
The procedure also requires that the nuts and washers be removed from the reactor vessel closure head and placed in storage racks during preparation for refueling. The storage racks are then removed from the refueling cavity and stored at convenient locations inside containment prior to reactor vessel closure head removal and refueling cavity flooding. After refueling and any other desired servicing, the reactor vessel closure head is replaced, and the components and systems are reassembled, generally by reversing the steps mentioned above.
Each procedure in the refueling process contributes significantly to the total cost associated with refueling the reactor. The total costs include costs associated with personnel time required to perform the refueling, power plant down time and consequent loss of electricity production, radiation exposure to personnel, and risks and costs associated with potential human errors. In addition, the various components that must be removed for refueling activities require a large amount of the limited storage space available inside containment and raise the risk of inadvertent contamination of work and storage areas.
Concepts and designs for integrating some of the reactor vessel closure head systems into a modular integrated head design have been proposed. For example, in U.S. Pat. No. 4,678,623 to Malandra et al., a head assembly is disclosed wherein vertical lift rods are attached to the reactor vessel lifting lugs and a missile shield, seismic support platform, CRDM cooling system, and lift rig are supported by the lift rods above the reactor vessel closure head. Because most or all of the modular head assembly taught by Malandra et al. is supported by the lift rods, however, very large loads are concentrated at the clevis connection at the reactor vessel closure head lifting lugs, which may cause damage to the lifting lugs and/or the body of the reactor vessel closure head. In addition, very heavy components, such as the missile shield and the fans, are supported at the distal ends of three relatively long lift rods, resulting in an unstable structure that may subject the lift rods to undesirable compressive, bending, and torsional stresses. Malandra et al. also does not provide a structure for putting a shroud around the CRDMs.
In U.S. Pat. No. 4,830,814, Altman discloses an integrated head package having a missile shield that is slidably mounted near the distal end of three lift rods connecting to the reactor vessel closure head lifting lugs. A shroud is shown disposed about the CRDMs. Similar to the apparatus disclosed by Malandra et al., however, the heavy missile shield and lifting rig are installed at the distal end of three elongate lift rods that are connected at their proximal end to the reactor vessel closure head lifting lugs. The Altman apparatus, therefore, will also produce relatively high local loads in the reactor vessel lifting lugs and head. Altman also does not disclose any system for cooling the CRDMs.
In U.S. Pat. Nos. 6,546,066 and 6,618,460, which are hereby incorporated in their entirety, the present inventor discloses an improved integrated head assembly having a cylindrical shroud on a ring support mounted to the reactor vessel closure head. The disclosed integrated head assembly includes a forced air cooling system, missile shield, and seismic support system. The integrated head assembly is removable in a single lift. In some applications, however, it may be preferable to have at least portions of the head assembly supported separately from the reactor head, and/or to divide the lift for removing these systems into more than a single lift.
There remains a need, therefore, for an integrated head assembly for a pressurized water reactor that can be removed from the reactor vessel integrally with the reactor vessel closure head and that does not introduce undue local stresses at the reactor vessel closure head and lifting lugs.